The present invention relates generally to chemical processing columns in which mass transfer and/or heat exchange between fluid streams occurs and, more particularly, to contact trays used in such columns to facilitate contact between fluid streams flowing within the column and methods of contacting the fluid streams using the contract trays.
Contact trays are used within mass transfer and heat exchange columns to facilitate contact between fluid streams flowing in countercurrent relationship within the column. The fluid streams are typically an ascending vapor stream and a descending liquid stream, in which case the contact trays are commonly referred to as vapor-liquid contact trays. In some applications, both fluid streams are liquid streams and the contact trays are commonly referred to as liquid-liquid contact trays. In still other applications, the ascending fluid stream is a gas stream and the descending fluid steam is a liquid stream, in which case the contact trays are referred to as gas-liquid contact trays.
The contact trays each have a planar tray deck on and above which interaction between the ascending fluid stream and the descending fluid stream occurs, a plurality of openings to allow upward passage of the ascending fluid stream through the tray deck and into the descending fluid stream to create a froth or mixture in which the desired mass transfer and/or heat exchange occurs, and at least one downcomer that directs the descending fluid stream from the associated tray deck to a tray deck on an underlying contact tray. The contact trays are positioned within the column in vertically spaced-apart relationship with each of the tray decks extending horizontally to fill the entire internal cross-section of the column.
One type of vapor-liquid contact tray developed for high fluid flow capacity and high efficiency applications uses a plurality of cylindrical cans on the tray deck to enhance the mixing of the vapor and liquid streams. The cylindrical cans extend upwardly from the tray deck and surround vapor openings formed in the tray deck. Swirl vanes positioned within each cylindrical can impart a swirling motion to the vapor ascending within the cylindrical can. A downcomer directs liquid from an overlying tray into the center of each cylindrical can where it is picked up by the swirling vapor to cause vigorous vapor-liquid interaction that leads to high separation efficiency and subsequent disengagement of the liquid phase from the vapor phase.
The centrifugal force imparted to the liquid by the swirling vapor within the cylindrical can causes the liquid to splash against and rise along the inner wall surface of the cylindrical can. Openings in the inner wall surface allow the liquid to pass through the inner wall surface and then descend onto the tray deck. The liquid travels along the tray deck and enters an opening in the tray deck that forms an inlet to a downcomer that directs the liquid into a cylindrical can on an underlying contact tray. The swirling vapor exits the cylindrical can through an open top of the can and then ascends into and through an opening surrounded by a cylindrical can in the tray deck of an overlying contact tray. In this manner, the vapor stream ascends and the liquid stream descends from can to can in successive contact trays.
In mass transfer and heat exchange columns, the efficiency of the vapor-liquid or liquid-liquid interaction suffers significantly if the fluid streams are not uniformly distributed across the internal cross section of the column. Such maldistribution can occur when the contact trays are not horizontally aligned, causing the liquid stream to channel towards the low side of the trays and the vapor stream to channel toward the high side of the trays. As a result of this channeling, the desired intermixing of the liquid and vapor streams is not achieved and the efficiency of the interaction suffers. This problem can result from improper installation of the contact trays, when land-based columns sway under heavy wind loads, or when offshore columns are positioned on floating platforms, barges or ships and are subject to a rocking motion as a result of wind and wave action.
A need thus exists for apparatus and methods to reduce the maldistribution and channeling of fluid streams that can occur when columns are subjected to rocking or swaying motion or when the contact trays positioned within the columns are otherwise tilted out of a horizontal orientation, such as through improper installation.